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Porous media matric potential and water content measurements during parabolic flight

Control of water and air in the root zone of plants remains a challenge in the microgravity environment of space. Due to limited flight opportunities, research aimed at resolving microgravity porous media fluid dynamics must often be conducted on Earth. The NASA KC-135 reduced gravity flight program offers an opportunity for Earth-based researchers to study physical processes in a variable gravity environment. The objectives of this study were to obtain measurements of water content and matric potential during the parabolic profile flown by the KC-135 aircraft. The flight profile provided 20-25 s of microgravity at the top of the parabola, while pulling 1.8 g at the bottom. The soil moisture sensors (Temperature and Moisture Acquisition System: Orbital Technologies, Madison, WI) used a heat-pulse method to indirectly estimate water content from heat dissipation. Tensiometers were constructed using a stainless steel porous cup with a pressure transducer and were used to measure the matric potential of the medium. The two types of sensors were placed at different depths in a substrate compartment filled with 1-2 mm Turface (calcined clay). The ability of the heat-pulse sensors to monitor overall changes in water content in the substrate compartment decreased with water content. Differences in measured water content data recorded at 0, 1, and 1.8 g were not significant. Tensiometer readings tracked pressure differences due to the hydrostatic force changes with variable gravity. The readings may have been affected by changes in cabin air pressure that occurred during each parabola. Tensiometer porous membrane conductivity (function of pore size) and fluid volume both influence response time. Porous media sample height and water content influence time-to-equilibrium, where shorter samples and higher water content achieve faster equilibrium. Further testing is needed to develop these sensors for space flight applications.

Effects of variable gravity on porous media matric potential and water content measurements

Control of water and air in the root zone of plants remains a challenge in microgravity. nDue to limited flight opportunities research aimed at resolving fluid dynamics in microgravity porous nmedia must often be conducted on earth. KC135 flight offers an opportunity for earth-based nresearchers to study physical processes in a variable gravity environment. The objectives of this nstudy were to obtain measurements of water content and matric potential during the parabolic profile nflown by the KC135 aircraft. The flight profile was designed to give 20-25 seconds of microgravity at nthe top of the parabola, while pulling 1.8-g at the bottom. Temperature and Moisture Acquisition nSensors (TMAS; Orbital Technologies, Madison, WI) use a heat-pulse method to measure water ncontent. Tensiometers were constructed using a porous membrane with a pressure transducer and nwere used to measure matric potential. The two types of sensors were placed at different depths in na substrate compartment filled with 1-2 mm Turface (calcined clay). The TMAS sensors were unable nto monitor bulk changes in water content in the substrate compartment, but were able to track local nmoisture changes in the soil profile. There were differences in water content data recorded at zero-, none-, and 1.8-g, but these were not significant. Tensiometer readings tracked pressure differences ndue to the hydrostatic force changes with variable gravity. The readings may have been affected by nchanges in cabin air pressure that occurred during each parabola. Tensiometer porous membrane nconductivity (function of pore size) and fluid volume both influence response time. Porous media nsample height and water content influence time-to-equilibrium, where shorter samples and higher nwater content achieve faster equilibrium. Further testing is needed to develop these sensors for nspace flight applications.